feat(platform): add priority-classes app with low/power/medium/high classes (#174)

## Summary
- New `apps/base/priority-classes/` app with four `PriorityClass` objects managed via the `platform` ArgoCD project
- Adds `apps/overlays/*/priority-classes` to the platform ApplicationSet generator
- Adds `priority-classes` namespace to platform AppProject destinations (required even for cluster-scoped resources)

| Class | Value | PreemptionPolicy | Intent |
|---|---|---|---|
| `low` | 100 | Never | Background work; evictable, won't preempt others |
| `power` | 100 | Never | Compute-heavy but expendable (e.g. AI/ML workloads) |
| `medium` | 10000 | PreemptLowerPriority | Standard services |
| `high` | 100000 | PreemptLowerPriority | Critical services; preempts lower-priority pods |

`PriorityClass` is already in the platform project's `clusterResourceWhitelist` so no project policy changes were needed.

## Test plan
- ArgoCD syncs `platform-priority-classes` successfully
- `kubectl get priorityclasses low power medium high` shows all four classes

Reviewed-on: #174

argocd/applicationsets/platform.yaml

@@ -22,6 +22,7 @@ spec:
       - path: apps/overlays/*/jfrog
       - path: apps/overlays/*/kanidm
       - path: apps/overlays/*/node-feature-discovery
+      - path: apps/overlays/*/priority-classes
       - path: apps/overlays/*/puppet
       - path: apps/overlays/*/purelb
       - path: apps/overlays/*/reflector-system